Creep-compensating strain gages

ABSTRACT

Creep effects exhibited by elastically deformable load sensing members equipped with foil-type resistive strain gages are counteracted by way of strain gage constructions in which a plurality of relatively short end-to-end grids, including a multiplicity of end tabs, develop compensatory inversely varying electrical output characteristics due to slippage.

United States Patent 91 Paetow [54] CREEP-COMPENSATING STRAIN GAGES [75]Inventor: Jiirgen Paetow, 61 Darmstadt, Germany [73] Assignee: HottingerBaldwin Messtechnik GmbH, Darmstadt, Germany [22] Filed: Oct. 12, 1971[21] Appl.No.:188,305

[30] Foreign Application Priority Data Oct. 10, 1970 Germany ..P 20 49820.8

[52] US. Cl. ..338/2 [51] Int. Cl. ..G01l 1/22 [-58] Field of Search..338/26;

73/885 R, 88.5 SP, 358 AR [4 1 June 5, 1973 Primary Examiner-C. L.Albritton Attorney-James E. Mrose and Mary C. Thomson etal. v

[57] ABSTRACT Creep effects exhibited by elastically deformable loadsensing members equipped with foil-type resistive strain gages arecounteracted by way of strain gage constructions in which a plurality ofrelatively short end-to-end grids, including a multiplicity of end tabs,

develop compensatory inversely varying electrical output characteristicsdue to slippage.

12 Claims, 4 Drawing Figures Patented June 5, 1973 v 3,737,827 I FV'RIORART 6 i 1 CREEP-COMPERSTATING STRAIN GAG-ES BACKGROUND OF THE INVENTION-A problem of long standing in the strain gage trans ducer art has beenthat of so-called creep, in the elasticallydeform-ablematerials:(example: certain-steels) of which the sensing elements ofsuch et'ransduce'rs are made. Whether the sensing elements are in theformof columns, beams, diaphragms shafts, orother linear, ra-

dial or angular force-translating configurations, these unwanted effectsare fundamentally the same, namely that-the elastically defo'rmablematerial tends toexhibit somewhat sustained deformation in the course oftime under prolonged stressconditions. Likewise, similar creep occurs inthe backings, adhesive bonds, and filamer ts of conventional,resistance-type strain :gages which may be applied to the underlyingsensing-elm mentsundergoing stress, and, together, these "effects maybecharacterized as the material creep in a gaged structure.

When the underlying sensing element is subjected to stress of loading,the electrical output promoted by an associated strain gage bridge tendsto=be relatively "high at first, and then to decrease to. a lower levelasthe loading persists; and, when the loading-is suddenlyre moved,.thebridge outputtends to'go negative andthen to drift slowly back towardthe expected zero. Seem- 'ingly, these varying erroneous :outputcharacteristics areof the wrong sensefbecause the aforesaid material"creep would, by itself, dic'tate the opposite behavior 7 wherein theoutput would'rise gradually to a predetermined level when load isapplied and would decrease gradually toward zero when loadis removed.The dif ferences appear to be accounted for bya collateral stress-isparticularly high because of the-abruptness of material'changes. The end'-tabs'or interconnections be tween adjacent filaments in astrain-:gage-grid are disposed at these .positicn'sfof highshearstressofcourse, a

but there can be significant slippagedespite the locking actions ofthese tabs. I

Effegts of the aforementioned slippage =upon electriduced by materialcreep; and, 'in theicaseof wire gages,

the minute end tabs tendto slip so; significantly thatrtheslipinfluences predomijnate: and. the net electrical output variationsvof the assemblyareas. described above.

I The situation involving foil-type gages-is some-whardi-fferent, inthat the flat erid. tabs. or interconnections bee tween adjacent flatfilaments are of relatively broad cal output of a gage arethe-reverse ofthe 'effects'in- 350 area and tend to sli'p relativelylittle.Consequently, the

effects of material: creep-predominate; and thenet electricfal outputs.experienced with foil gages: tend-to rise gradually to apredetermined-levelwhen; loadis. appliedancl to decreasegradually-toward zero whenload is re-- moved. It is to. thecompensationrof theseeffectscf material' creep that the present.invention is particularly di' rected, with a principal; objective beingto yield a net electrical out-put characteristic-in which nei-therslipnor creep.- variations will predominate. and: in which, in-

stead, the output will at once rise to and remain at a fixed levelwhen apredetermined load is applied and will at once decrease'to and remain atsubstantially zero level when the load is suddenly removed.

SUMMARY The presentinvention is aimed at creating improved straingageinstallationsin which compensations for net electrical output variationsdue to material creep effects are developed by way of unique'gridconfigurations of the gage filaments. I

Based upon" distinctive recognitions that shear stresses in aresistance-type strain gage are a maximum atlocationsofdiscontinuitiesinthe'filaments along the direction of strains to be measured, and thatthe effects of gage filament slippages will be enhanced by numerous suchdiscontinuities in relation to the overall lengths and resistances ofthe filaments, gages are formed with numerous short-length filamentshaving locking elements, such as end tabs, disposed at maximum-shear-stress positions at the sites'of the discontinuities.

By way of a summary account of practice of this invention in oneofitsaspects, numerous'relatively short filaments of a foil-typeresistance strain gage are disposed in spaced end-to-end relationship inalignment with direction of strains to be' measured, the short filamentsbeing subdivided into'serially-connected grid arrays. For uses where thestress gradient in the underlying sensing element'is non-uniform, minthe caseof the side of a holefin such an element, numerousend tabs forthe short filaments are advantageously disposed at the locus of peakedstress. Electrical connections providing alternate "points forelectrical tapping of the gage outputs, to selectively include orexclude one or more auxiliary grids which exhibit desired slip, affordconvenient in situs adjustment of creep compensation of a gageinstallation.

BRIEF DESCRIPTION OF THE DRAWING ment of FIG. 3 in which further gridsof even shorterlength are serially added, together with alternateelectrical'connections,the added grids providing means foradjustingoverall creep compensation- DESCRIPTION OF THE PREFERREDEMBODIMENTS I 'Ihe'strain gage pattern appearing inFIG. 1 is' typical"of that of a conventional resistance ty e wire or foil gage, the usualbacking material, such as plastic, and adhesive bonding material andunderlying sensing element not being illustrated. Classically, theelongated filaments, 5, are substantially parallel and are con-:

neetedlelectrically in series into a single grid by way of end tabs orconnections 6. Electrical connections 7 for the endsbof the seriallyconnected filaments are shown tions 7C are provided, and the user maythereby select such as those wherein the gage is bonded along the Icurved interior of a sensing element having an accommodating hole, thestress gradient may be non-uniform and peaked about midway along thelength of the gage filaments.

For reasons already discussed hereinabove, the filaments of a generalcounterpart of the FIG. 1 gage appearing in FIG. 2 are at least in partmade up of shorterlength filaments 5a, 5b, 5c and 5d, these beingdisposed in smaller co-planar grid subdivisions 8, 9, l and 11,respectively, symmetrically alongside the longer centrally disposedfilaments Preferably, the conductive portions of the gage are of foilform, whether mechanically or electrochemically created, and the endtabs or interconnections 6', 6a, 6b, 6c and 6d are therefore ofrelatively broad area. As was explained hereinabove, these end tabs aredisposed at end positions where maximum shear stress is developed in thegage unit itself, and this is also true at those positions where thelongitudinally spaced end tabs 6a and 6b confront one another. Thelatter positions are sites of maximum shear stress because of thediscontinuities resulting from the longitudinal spacing between theconfronting end tabs. Each of the shorter-length filaments is less thanhalf the length of the length of filaments 5, the latter being whatwould normally be selected for the filament length for a particular gageinstallation.Cdiisequently,

the creep-compensating slippages in the short-length grid subdivisions8ll are larger in relation to the filament resistances'involved thanwould be the case with longer filaments, such as filaments all the samelength as filaments 5. Electrical connections 7A serve the same functionas connections 7 in FIG. 1.

In FIG. 3, the gage pattern involves only short-length filaments, tomaximize the aforementioned creepcompensating effects. There, the widershort-length grid subdivision 12, involving filaments 5e and end tabs 6eis longitudinally spaced a short distance from likelength narrower gridsubdivisions 13, involving filaments 5f and end tabs 6f, and gridsubdivision 14, involving filaments 5g and 6g. Connections 7B arefunctionally like 7 and 7A.

The gage arrangement in FIG. 4 corresponds in part to that in FIG. 3,and like portions thereof are correspondingly numbered, withdistinguishing single-prime accents applied. However, two further gridsubdivisions, l5 and 16, are added electrically in series with theterminal ends of grid subdivisions l3 and l4', respectively. Thesefurther grid subdivisions involve only relatively short filaments, 5hand Si, respectively, with end tabs 6h and 6i spaced in the direction ofthe strain an optimum series electrical combination of the gridsubdivisions by making terminal connections with appropriate ones ofconnections 7B and 7C.

The gages shown in'FIGS. 2-4 are all provided with extra end tabsabout'midway of their overall lengths, and will therefore developoptimum compensatory effects when responding to the aforementionednonuniform gradients of stresses peaking at about that mid location. Inother arrangements, the end tabs maybe disposed other than midway, ofcourse, and the grid subdivisions may be other than symmetrical.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A resistance-type filamentary strain gage for the compensation ofeffects of material creep, comprising a unitary assembly of a pluralityof substantially copla-' nar grid portions each including a plurality ofsubstantially parallel filaments serially interconnected by end 2. Aresistance-type filamentary strain gage as set forth in claim 1 whereinsaid filaments and end tabs comprise substantially flat foil, said endtabs being enlarged in area in relation to said filaments, and whereinsaid filaments are substantially aligned with the direction of strain tobe measured by said assembly.

. 3. A resistance-type filamentary strain gageas set forth in claim 2wherein each of said grid portions comprises an array of substantiallyequal-length foil filaments forming a substantially rectangularpattern,and wherein the assembly of said grid portions forms a substantiallyrectangular pattern. 1

4. A resistance-type filamentary strain gage as set forth in claim 2further comprising at least one additional grid portion connected inseries with one of said two electrical terminals and a third one of saidelectrical terminals.

5. A resistance-type filamentary strain gage as set forth in claim 4wherein said additional grid portion includes foil filaments of shorterlength than the filaments of said grid portions.

6. A resistance-type filamentary strain gage as set forth in claim 4wherein end tabs connecting adjacent ones of the filaments along one endof said additional grid portion are disposed at a position wheresubstantially the highest stress concentrations are to be developed in asensing element to which the gage is designed to be applied.

7. A resistance-type filamentary strain gage as set forth in claim 3wherein said confronting end tabs are disposed at a position wheresubstantially the highest stress concentrations are to be developed in asensing element to which the gage is designed to be applied.

8. A resistance-type filamentary strain gage as set forth in claim 3wherein said confronting end tabs are disposed substantially midwayalong the length of the assembly in the strain-sensing direction.

9. A resistance-type filamentary strain gage as set forth in'claim 3wherein said grid portions are disposed symmetrically about a midposition aligned with said direction of strain.

10. A resistance-type filamentary strain gage for the compensation ofeffects of material creep, comprising a unitary assembly of a bondingmaterial and a plurality of substantially coplanar filaments, at leastsome of said filaments being in substantially end-to-end relationshipswith longitudinal spacing the-rebetween, end tabs at the ends of theend-to-end filaments which are more remote from said longitudinalspacing and serially interconnecting adjacent ends thereof,'and meanselectrically connecting ends of said filaments at the site of saidlongitudinal spacing while at the same time anchoring said ends withsaid bonding material, electrical terminals for said assembly, and meanselectrically connecting the interconnected filaments in series betweentwo of said electrical terminals.

11. A resistance-type filamentary strain gage as'set forth in claimwherein said filaments and end tabs and connecting means comprisesubstantially flat foil,

and wherein said filaments are substantially aligned with the directionof strain to be measured by said assembly.

12. A resistance-type filamentary strain gage for the compensation ofeffects of material creep, comprising a unitary assembly of a firstsubstantially parallel group nals.

1. A resistance-type filamentary strain gage for the compensation ofeffects of material Creep, comprising a unitary assembly of a pluralityof substantially coplanar grid portions each including a plurality ofsubstantially parallel filaments serially interconnected by end tabs, atleast some of the filaments of different ones of said grid portionsbeing in endto-end relationship with respective confronting end tabsthereof spaced from one another, electrical terminals for said assembly,and means electrically connecting said grid portions in series betweentwo of said electrical terminals.
 2. A resistance-type filamentarystrain gage as set forth in claim 1 wherein said filaments and end tabscomprise substantially flat foil, said end tabs being enlarged in areain relation to said filaments, and wherein said filaments aresubstantially aligned with the direction of strain to be measured bysaid assembly.
 3. A resistance-type filamentary strain gage as set forthin claim 2 wherein each of said grid portions comprises an array ofsubstantially equal-length foil filaments forming a substantiallyrectangular pattern, and wherein the assembly of said grid portionsforms a substantially rectangular pattern.
 4. A resistance-typefilamentary strain gage as set forth in claim 2 further comprising atleast one additional grid portion connected in series with one of saidtwo electrical terminals and a third one of said electrical terminals.5. A resistance-type filamentary strain gage as set forth in claim 4wherein said additional grid portion includes foil filaments of shorterlength than the filaments of said grid portions.
 6. A resistance-typefilamentary strain gage as set forth in claim 4 wherein end tabsconnecting adjacent ones of the filaments along one end of saidadditional grid portion are disposed at a position where substantiallythe highest stress concentrations are to be developed in a sensingelement to which the gage is designed to be applied.
 7. Aresistance-type filamentary strain gage as set forth in claim 3 whereinsaid confronting end tabs are disposed at a position where substantiallythe highest stress concentrations are to be developed in a sensingelement to which the gage is designed to be applied.
 8. Aresistance-type filamentary strain gage as set forth in claim 3 whereinsaid confronting end tabs are disposed substantially midway along thelength of the assembly in the strain-sensing direction.
 9. Aresistance-type filamentary strain gage as set forth in claim 3 whereinsaid grid portions are disposed symmetrically about a mid positionaligned with said direction of strain.
 10. A resistance-type filamentarystrain gage for the compensation of effects of material creep,comprising a unitary assembly of a bonding material and a plurality ofsubstantially coplanar filaments, at least some of said filaments beingin substantially end-to-end relationships with longitudinal spacingtherebetween, end tabs at the ends of the end-to-end filaments which aremore remote from said longitudinal spacing and serially interconnectingadjacent ends thereof, and means electrically connecting ends of saidfilaments at the site of said longitudinal spacing while at the sametime anchoring said ends with said bonding material, electricalterminals for said assembly, and means electrically connecting theinterconnected filaments in series between two of said electricalterminals.
 11. A resistance-type filamentary strain gage as set forth inclaim 10 wherein said filaments and end tabs and connecting meanscomprise substantially flat foil, and wherein said filaments aresubstantially aligned with the direction of strain to be measured bysaid assembly.
 12. A resistance-type filamentary strain gage for thecompensation of effects of material creep, comprising a unitary assemblyof a first substantially parallel group of foil filaments and at leastone further group of substantially parallel foil filaments, said foilfilaments of said further group being less than about half the length ofthe filaments of said first group and being substaNtially paralleltherewith, electrical terminals for said assembly, end tabs connectingadjacent ends of said filaments, and means electrically connecting allof said filaments in series between two of said electrical terminals.